Syrian hamster Female Protein (FP) was shown to be a sex limited pentraxin which bound to phosphocholine in Ca ions dependent fashion and was structurally homologous to amyloid P component and C-reactive protein. The immunological deficiency of hamster spleen may be explained by a nonspecific suppressor cell which can be separated by differential adherence to nylon wool. The cotton rat (Sigmodon hispidis/NIH) was found to have an inverted IgG1/IgG2 serum concentration ratio, which may explain its unique usefulness in respiratory syncytial virus studies. A mutant strain of Lewis rats was genetically resistant to development of experimental allergic encephalomyelitis although hypersensitivity was present.